A piezoelectric acoustic module such as a piezoelectric speaker for use in a mobile telephone or the like is intended for sound amplification for audio signals and, therefore, a piezoelectric element vibrates with a large amplitude and thus is conventionally supported by a material with high mechanical compliance. A problem of this structure is that while molding is carried out for ensuring water-tightness, when molded, the temperature characteristics are poor. In particular, if molding is carried out using a material with high flexibility such as polyurethane, the flexible material hardens at a low temperature so that the resonance frequency in the form of a molded body increases, while, the flexible material softens at a high temperature so that the resonance frequency in the form of a molded body decreases. Consequently, the temperature dependence of the resonance frequency is high, resulting in difficulty for practical use.
Patent Document 1 discloses an acoustic vibration generating element with improved temperature dependence, wherein a silicone rubber solution is poured into a brass mold over the entire surfaces of a bimorph element and then a curing treatment is carried out to cover the bimorph element with a rubber coating film having a thickness of 2 mm on the two surfaces of the bimorph element in its thickness direction and a thickness of 1 mm in its width direction.